Subject: Re: [PATCH v4 29/33] x86/mm: try VMA lock-based page fault handling first

From: Jiri Slaby <[email-masked]>
Date: Thu, 29 Jun 2023 16:40:44 +0200

Hi,

On 27. 02. 23, 18:36, Suren Baghdasaryan wrote:
> Attempt VMA lock-based page fault handling first, and fall back to the
> existing mmap_lock-based handling if that fails.
> 
> Signed-off-by: Suren Baghdasaryan <[email-masked]>
> ---
>   arch/x86/Kconfig    |  1 +
>   arch/x86/mm/fault.c | 36 ++++++++++++++++++++++++++++++++++++
>   2 files changed, 37 insertions(+)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index a825bf031f49..df21fba77db1 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -27,6 +27,7 @@ config X86_64
>   	# Options that are inherently 64-bit kernel only:
>   	select ARCH_HAS_GIGANTIC_PAGE
>   	select ARCH_SUPPORTS_INT128 if CC_HAS_INT128
> +	select ARCH_SUPPORTS_PER_VMA_LOCK
>   	select ARCH_USE_CMPXCHG_LOCKREF
>   	select HAVE_ARCH_SOFT_DIRTY
>   	select MODULES_USE_ELF_RELA
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index a498ae1fbe66..e4399983c50c 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -19,6 +19,7 @@
>   #include <linux/uaccess.h>		/* faulthandler_disabled()	*/
>   #include <linux/efi.h>			/* efi_crash_gracefully_on_page_fault()*/
>   #include <linux/mm_types.h>
> +#include <linux/mm.h>			/* find_and_lock_vma() */
>   
>   #include <asm/cpufeature.h>		/* boot_cpu_has, ...		*/
>   #include <asm/traps.h>			/* dotraplinkage, ...		*/
> @@ -1333,6 +1334,38 @@ void do_user_addr_fault(struct pt_regs *regs,
>   	}
>   #endif
>   
> +#ifdef CONFIG_PER_VMA_LOCK
> +	if (!(flags & FAULT_FLAG_USER))
> +		goto lock_mmap;
> +
> +	vma = lock_vma_under_rcu(mm, address);
> +	if (!vma)
> +		goto lock_mmap;
> +
> +	if (unlikely(access_error(error_code, vma))) {
> +		vma_end_read(vma);
> +		goto lock_mmap;
> +	}
> +	fault = handle_mm_fault(vma, address, flags | FAULT_FLAG_VMA_LOCK, regs);
> +	vma_end_read(vma);
> +
> +	if (!(fault & VM_FAULT_RETRY)) {
> +		count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
> +		goto done;
> +	}
> +	count_vm_vma_lock_event(VMA_LOCK_RETRY);

This is apparently not strong enough as it causes go build failures like:

[  409s] strconv
[  409s] releasep: m=0x579e2000 m->p=0x5781c600 p->m=0x0 p->status=2
[  409s] fatal error: releasep: invalid p state
[  409s]

[  325s] hash/adler32
[  325s] hash/crc32
[  325s] cmd/internal/codesign
[  336s] fatal error: runtime: out of memory

There are many kinds of similar errors. It happens in 1-3 out of 20 
builds only.

If I revert the commit on top of 6.4, they all dismiss. Any idea?

The downstream report:
https://bugzilla.suse.com/show_bug.cgi?id=1212775

> +
> +	/* Quick path to respond to signals */
> +	if (fault_signal_pending(fault, regs)) {
> +		if (!user_mode(regs))
> +			kernelmode_fixup_or_oops(regs, error_code, address,
> +						 SIGBUS, BUS_ADRERR,
> +						 ARCH_DEFAULT_PKEY);
> +		return;
> +	}
> +lock_mmap:
> +#endif /* CONFIG_PER_VMA_LOCK */
> +
>   	/*
>   	 * Kernel-mode access to the user address space should only occur
>   	 * on well-defined single instructions listed in the exception
> @@ -1433,6 +1466,9 @@ void do_user_addr_fault(struct pt_regs *regs,
>   	}
>   
>   	mmap_read_unlock(mm);
> +#ifdef CONFIG_PER_VMA_LOCK
> +done:
> +#endif
>   	if (likely(!(fault & VM_FAULT_ERROR)))
>   		return;
>   

thanks,
-- 
js
suse labs